1. Field of the Invention
The invention relates to an ultrasound therapeutic apparatus which irradiates a diseased part of a patient with focused ultrasound beam for therapy thereof.
2. Description of the Related Art
In recent years, attention has been paid to minimally invasive treatment (MIT) a part of which is performed by ultrasonic therapeutic apparatus. Ultrasonic (Ultrasound) therapeutic apparatuses include shock wave lighotriptor which break a calculus with a focused ultrasonic beam and thermotherapy apparatuses which heat and necrotize a diseased part, such as a cancer, with focused ultrasonic beam.
A typical example of a strong ultrasound generating device is a piezoelectric type of device. This type of ultrasound generating device has great advantages that the focus of ultrasonic waves can be localized, few expendable parts are involved, intensity control is easy, the position of the focus can be changed easily by phase control (delay control) of drive voltages to a plurality of piezoelectric transducer elements, etc. (refer to Japanese Unexamined Patent Publication No. 60-145131 and U.S. Pat. No. 4,526,168).
The MIT is also a key word in the field of cancer therapy. Under the present conditions, most of cancer therapies rely on surgical operations. Thus, the outward form and the inherent function of an organ having cancer are destroyed very frequently. In such a case, so much strain will be put on a patient even if he or she lives long after operation. From a viewpoint of quality of life (QOL), therefore, the development of a new therapy (equipment) that is little invasive is being desired.
Under such circumstances, as one of therapies for malignant tumors, or cancers, the therapy by hyperthermia has drawn attention, which, using a difference in sensitivity to heat between tumor tissues and normal tissues, selectively destroys only cancer cells by heating a diseased part to 42.5.degree. C. or more for a long period of time. As a method of application of heat to the body, a method of using electromagnetic waves such as microwaves has preceded. With this method, however, the electrical characteristics of a living body make it difficult to selectively heating a tumor in the deep of the body. Satisfactory results cannot therefore be expected for tumors existing 5 cm or more deep in the body. For this reason, a method of utilizing ultrasonic energy has been proposed for therapy for tumors existing in the deep of the body (refer to Japanese Unexamined Patent Publication No. 61-13955).
The ultrasound-based thermotherapy has been developed into a therapy which, by sharply focusing ultrasonic waves generated by piezoelectric transducer elements onto a diseased part, heats a tumor to 80.degree. C. or more and necrotizes tumor tissues in an instant (refer to U.S. Pat. No. 5,150,711). In this therapy, unlike the conventional hyperthermia, it is a very important subject to precisely match the focus or point of application of focused ultrasonic waves with a diseased part in order to introduce ultrasonic waves at a very great intensity (some hundreds to some thousands of W/cm.sup.2) into a restricted region in the vicinity of the focus of the ultrasonic waves and necrotize the diseased part instantly.
Methods of solving that problem are disclosed in Japanese Unexamined Patent Publications Nos. 61-13954, 61-13956, and 60-145131. According to these methods, the spatial intensity distribution of therapeutic ultrasonic waves is obtained by first detecting by an imaging probe echoes from the focus region of the waves pulses emitted from a therapeutic ultrasonic source, and then performing a B-mode process on the received echo signal.
However, these methods have the following problem. Whereas the frequency of the therapeutic ultrasonic waves is in the range of 1 to 3 MHz, the frequency of in vivo imaging ultrasonic waves is 3.5 MHz or more. The resonant frequency of imaging transducer elements coincides with the frequency of the imaging ultrasonic waves. Thus, the imaging probe will receive echoes of therapeutic ultrasonic waves with a very low sensitivity, failing to obtain the intensity distribution with precision.
According to one of these methods, an imaging probe receives echoes of imaging ultrasonic waves generated by that probe and echoes of ultrasonic pulses at the same time. The received signal having two components mixed in is then processed. Thus, this method has a problem that each image contrast cannot be adjusted individually.
According to another of these methods, imaging ultrasonic waves and ultrasonic pulses for intensity distribution are transmitted/received alternately. This approach has an advantage that each image contrast can be adjusted individually, but has a disadvantage that the frame rate is low, making it difficult to accomplish real-time processing.
Moreover, the above methods have the following problem. In general, in cautery treatment based on focusing of high-intensity ultrasonic waves, the location of a region onto which ultrasonic waves are focused and the location of a region to be treated do not match due to changes in acoustic characteristic of a cauterized region. Thus, even if the focus is matched to the part to be treated, the therapy will result in imperfection. Moreover, an adverse effect will also be produced on normal parts.
Furthermore, the above methods have the following problem. Ultrasonic pulses generated from a source of therapeutic ultrasonic waves are reflected from a region in the vicinity of the focus and the resulting echoes are received by an imaging probe. Likewise, imaging ultrasonic waves generated from the imaging probe are reflected from a region in the vicinity of the focus and the resulting echoes are received by the imaging probe. In order to cause a match to occur between the location of the focus on an intensity distribution image and the location of the focus on a B-mode tomographic image, the therapeutic ultrasonic waves and the imaging ultrasonic waves are generated at different times so that the resulting echoes corresponding to both types of waves from the focus will arrive at the probe at the same time. However, the timing of receiving of each echo from the same location other than the focus differs because of differences in propagation path. Thus, a spatial mismatch occurs between an intensity distribution image and a B-mode tomographic image.